1. Field of the Invention
The present invention generally relates to an ink jet head and a method of producing the same. More particularly, the present invention relates to an ink jet head that discharges ink droplets by driving a diaphragm with electrostatic force between electrodes on the diaphragm side and electrodes that face the diaphragm-side electrodes, and to a method of producing such an ink jet head.
2. Description of the Related Art
Japanese Laid-Open Patent Application No. 7-125196 discloses an electrostatic ink jet head that comprises a diaphragm, a substrate integrally formed with the diaphragm, and individual electrodes that face the diaphragm, with a gap being interposed between the diaphragm and the individual electrodes. In this ink jet head, the individual electrodes are formed in concave portions formed in an insulating member, and the gap is defined as: (the depth of each concave portion of the insulating member (or the height of each step portion))xe2x88x92(the thickness of each individual electrode). The individual electrodes and the diaphragm electrodes can be pulled out on the same planes, respectively, so that electric voltage can be applied to them.
Japanese Patent Application No. 9-148062 discloses an electrostatic ink jet head that comprises a diaphragm and individual electrodes which face the diaphragm, with a gap being maintained between the diaphragm and the individual electrodes. In this ink jet head, the individual electrodes are formed in concave portions formed in a glass substrate, and the gap is defined as: (the height of each step of the glass substrate)xe2x88x92(the thickness of each individual electrode). Through holes for embedding conductors in the glass substrate are formed, and conductors are embedded in the through holes. The individual electrodes are pulled onto the bottom surface of the glass substrate through the conductors, and are mounted via bump-like conductors. A voltage is then applied.
Japanese Patent Application No. 10-61308 discloses an electrostatic ink jet head in which individual electrodes are formed by a diffusion layer in a silicon substrate. Through holes for pulling out the electrodes onto the silicon substrate are formed, so that the potential of the individual electrodes can be taken out onto the bottom surface of a supporting substrate. After the formation of the through holes, the electrodes are formed by the diffusion layer.
Japanese Laid-Open Patent Application No. 5-50601 discloses an electrostatic ink jet head in which the diaphragm is deformed by electrostatic force generated by a voltage applied between the diaphragm and electrodes facing the diaphragm, thereby discharging ink droplets. Each diaphragm chamber (gap) is formed by a concave portion in a diaphragm substrate. A lower substrate (electrode substrate) also has concave portions. The individual electrodes are placed in the concave portions, so as to prevent short-circuiting with the diaphragm.
Japanese Laid-Open Patent Application No. 6-71882 discloses an electrostatic ink jet head in which the gap between the diaphragm and each facing electrode is in the range of 0.05 xcexcm and 2.0 xcexcm, so that the ink jet head can be driven at a low voltage. More specifically, electrodes are place din concave portions formed in at least one of an electrode substrate or a diaphragm substrate. Accordingly, the gap length is determined by the difference between the depth of each concave portion and the thickness of each electrode. The electrodes are formed by a diffusion layer in a silicon substrate. In this case, the gap length is determined by the thickness of an oxide film formed as a gap spacer.
Japanese Laid-Open Patent Application No. 9-193375 discloses an electrostatic ink jet head in which each gap between the diaphragm and electrodes facing the diaphragm has a non-parallel shape so as to restrict variations of the discharging amount and the discharging rate of ink droplets. Furthermore, the diaphragm and the individual electrodes facing the diaphragm are bonded via an insulating coating layer, so that a collision between the diaphragm ad the individual electrodes can be avoided. Each gap is formed between a step portion or a concave portion in the diaphragm substrate and a non-parallel step portion of the electrode substrate.
An ink jet head of an electrostatic actuator type in which the diaphragm is deformed by electrostatic force so as to generate pressure wave in an ink chamber can be produced by a wafer process. Accordingly, the ink jet head can have high density and a large number of stable devices can be produced. The ink jet head having a planar structure can be made smaller, as disclosed in Japanese Laid-Open Patent Application No. 7-125196 and others. The diaphragm is vibrated by electrostatic force caused by a voltage applied between the diaphragm and the individual electrodes and by the rigidity of the diaphragm. With the vibration of the diaphragm, ink is sucked in and discharged.
The pulling out of the individual electrodes disclosed in Japanese Laid-Open Patent Application 7-125196 is carried out on parts of the surface of the electrode substrate, with which neither liquid chamber nor diaphragm substrate is in contact. As disclosed in Japanese Patent Application Nos. 9-148062 and 10-61308, the individual electrodes are pulled out from the bottom side of the electrode substrate, so that the chip area and the number of mounting steps can be reduced.
As disclosed in Japanese Laid-Open Patent Application No. 7-125196 and Japanese Patent Application No. 9-148062, concave portions are formed in an insulating substrate, and electrodes made of a conductive material such as metal are placed in the respective concave portions, thereby obtaining the individual electrodes. As disclosed in Japanese Patent Application 10-61308, the individual electrodes may also be constituted by conductive impurity (dopant) diffusion regions formed in a silicon substrate.
The displacement xc3xa4(m) of the diaphragm of the electrostatic ink jet head is determined by the equation (1), and the electrostatic attraction P (N/m2) is determined by the equation (2).
xc3xa4=kxc3x9712(1xe2x88x92v2)/Eh3xc3x97Pa4xe2x80x83xe2x80x83(1)
P: electrostatic attraction (N/m2)
a: short side length (m)
h: diaphragm thickness
v: Poisson""s ratio
E: Young""s modulus
k: constant
P=xc2xdxc3x97{dot over (xc3xa5)}xc3x97(V/Geff)2xe2x80x83xe2x80x83(2)
{dot over (xc3xa5)}: dielectric constant (F/m)
V: voltage (V)
Geff: effective gap length (m)
In accordance with the above equations, the displacement of the diaphragm due to electrostatic force is inversely proportional to the square of the effective gap length Geff. Therefore, it is important to form the gaps at high precision. Also, the effective gap length Geff needs to be made smaller so as to have a low driving voltage. In other words, it is necessary to form narrow gaps at high precision.
On the other hand, in the case where the individual electrodes are formed in the concave portion in an insulating substrate (or in an insulating film on a conductor or a semiconductor substrate), as disclosed in Japanese Laid-Open Patent Application No. 7-125196 and Japanese Patent Application No. 9-148062, the effective gap length Geff can be expressed as:
Geff=(concave depthxe2x88x92individual electrode thickness)xe2x80x83xe2x80x83(3)
In this equation, the passivation film or insulation film on the individual electrodes is no taken into consideration. As is apparent from the equation (3), the effective gap length Geff is influenced by both variations of the depth of the concave portions and the thickness of the individual electrodes. If the following relationship (4):
concave depth greater than individual electrode thicknessxe2x80x83xe2x80x83(4)
is satisfied, the effective gap length Geff is determined mainly by the concave depth, which is relatively controllable. If the effective gap length Geff is small, the relationship (4) can be satisfied, as long as the thickness of the individual electrodes is very small. However, there is a limit to the thinness of the individual electrodes, in terms of resistance and workability. If the effective gap length Geff is made smaller to obtain a lower voltage, the control of variation of the effective gap length Geff becomes difficult.
In the case where the individual electrodes are constituted by the impurity (dopant) diffusion region in the silicon substrate, the effective gap length Geff is determined only by the concave depth, and may be narrowed to obtain a lower voltage. However, this structure requires sophisticated and complicated production procedures so as to ensure high voltage resistance between the substrate and the electrodes and between adjacent electrodes, and to reduce leakage current. As a result, the production costs are increased, and a voltage of only one polarity can be applied to the individual electrodes.
In the case where the electrodes are formed on an insulating substrate or on an insulating film on a substrate, as in the disclosures of Japanese Laid-Open Patent Application Nos. 5-50601 and 6-71882, the gap length is the difference between the electrode thickness and the depth of the concave portions formed in the diaphragm substrate and/or the electrode substrate. If the gap length is great and the electrode thickness is smaller than the concave depth, the precision of the gap length is determined mainly by the precision of the concave depth. However, if the gap length is made smaller so as to obtain a lower voltage, the variation of the gap length becomes greater. It is of course possible to restrict the variation of the gap length by reducing the thickness of the electrodes. In such a case, however, the resistance of the electrodes becomes high, and the driving voltage cannot be increased.
Japanese Laid-Open Patent Application No. 6-71882 discloses the structure having electrodes formed by a diffusion layer in the silicon substrate. In this structure, the precision of the gap length is not lowered by the thickness of the electrodes. However, since the electrodes and the substrate is separated by pn junction, a voltage of only one polarity can be applied, the process of ensuring enough voltage resistance is complicated, and it is difficult to maintain the yield of the pn junction with the electrodes having relatively large areas. Also, each diaphragm chamber and its surrounding area are not completely sealed. Therefore, it is necessary to perform a sealing step using a sealing member to protect the ink jet head from foreign matter during an actual operation. Still, there is a possibility that foreign matter will enter the ink jet head prior to the sealing step during the production procedure.
Japanese Laid-Open Patent Application No. 9-193375 discloses a structure in which the diaphragm is bonded to a part of the individual electrodes via an insulating coating layer. However, the bonding is not made on a single plane. Therefore, it is necessary to adjust the irregular surfaces of both substrates at the time of bonding. Even after the adjustment of the bonding surfaces, there will be small gaps between the bonding surfaces, resulting in poor bonding strength. Moreover, Japanese Laid-Open Patent Application No. 9-193375 does not teach specific materials and methods for boding.
It is a general object of the present invention to provide ink jet heads and methods of producing the same, in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide an electrostatic ink jet head in which the individual electrodes have high voltage resistivity, voltage of both polarities can be applied, and gap formation can be carried out with high precision through simpler production steps.
Another specific object of the present invention is to provide an electrostatic ink jet head in which the diaphragm and the electrode substrate are bonded to each other on the same plane, and the individual electrodes are bonded to the diaphragm via an insulating layer. With this structure, even if the gap length is short, the precision in gap formation can be high.
Further specific object of the present invention is to provide an electrostatic ink jet head in which the diaphragm is combined with the individual electrodes and the common electrode, so that the entire structure and the production steps can be dramatically simplified.
The above objects of the present invention are achieved by an electrostatic ink jet head, comprising: a plurality of nozzles through which ink droplets are discharged; a plurality of ink passages that communicate with the nozzles; a diaphragm that forms a part of each of the ink passages and has a common electrode; a plurality of individual electrodes that face the diaphragm; and spacers, each of which maintains a gap between the diaphragm and each of the individual electrodes. In this electrostatic ink jet head, a driving voltage is applied between the common electrode and the individual electrodes, so that the diaphragm is deformed by electrostatic force to pressurize ink in the ink passages. Also, in this electrostatic ink jet head, at least a part of the spacer is made of the same material as the individual electrodes.
Since the diaphragm and at least a part of the spacer that forms the gap between the diaphragm and each individual electrode is made of the same material as the individual electrodes, the gap formation can be carried out at high precision.
In the above electrostatic ink jet head of the present invention, the individual electrodes and a part of the spacer are made of monocrystal silicon, and the remaining part of the spacer is formed from silicon oxide film. With this structure, the gap formation can be carried out at higher precision.
The above electrostatic ink jet head of the present invention further comprises: an electrode supporting substrate that supports the individual electrodes; through holes, each of which penetrates through the electrode supporting substrate from a bottom side surface thereof to each corresponding individual electrode; and electrode retrieve pads formed on the bottom side surface of the electrode supporting substrate. With this structure, the chip size and the cost for packaging can be reduced.
In the above electrostatic ink jet head of the present invention, the electrode supporting electrode is a  less than 110 greater than  silicon substrate, and a surface of each of the through holes has a (111) plane. With this structure, high-density through hole formation can be carried out at high precision.
In the above electrostatic ink jet head of the present invention, the electrode supporting substrate is a silicon substrate that has a higher dopant concentration than that of the individual electrodes. With this structure, the oxidation speed is increased due to the impurities, and because of that, the through hole formation can be easily carried out at high precision.
The above objects of the present invention are also achieved by a method of producing an electrostatic ink jet head that comprises: a plurality of nozzles through which ink droplets are discharged; a plurality of ink passages that communicate with the nozzles; a diaphragm that forms a part of each of the ink passages and has a common electrode; a plurality of individual electrodes that face the diaphragm; and spacers, each of which maintains a gap between the diaphragm and each of the individual electrodes, the diaphragm being deformed by electrostatic force to pressurize ink in the ink passages. This method comprises the steps of: oxidizing an SOI substrate that is used as an electrode supporting substrate; performing etching on a part of a resultant oxide film; and performing etching to form a separation groove between each of the individual electrodes and each corresponding one of the spacers.
In accordance with this method of the present invention, the regular semiconductor production processes can be applied to the production of the electrostatic ink jet head. Thus, a highly reliable electrostatic ink jet head can be produced at high precision and at low costs.
The above method of the present invention further comprises the step of forming through holes after the formation of a material to be the individual electrodes on the electrode supporting substrate. With this method, the individual electrodes can be prevented from having through holes, thereby allowing more freedom in design.
The above objects of the present invention are also achieved by an ink jet head that discharges ink droplets through nozzles by deforming a diaphragm by electrostatic force caused by a driving voltage applied between a common electrode formed on the diaphragm and individual electrodes. This ink jet head comprises: a liquid chamber substrate provided with the diaphragm that forms a part of each of liquid chambers communicating with the nozzles; an electrode substrate having the individual electrodes facing the diaphragm via a gap. In this ink jet head, the liquid chamber substrate and the electrode substrate are bonded to each other on the same plane, with an insulating layer being interposed therebetween.
With the above structure, the gap formation can be carried out at high precision.
In the above ink jet head, the liquid chamber substrate and the electrode substrate are both made of monocrystal silicon, and the insulating layer interposed between the liquid chamber substrate and the electrode substrate is formed from silicon oxide film.
With this structure, less deformation is caused at the time of bonding, and the bonding region between the diaphragm and the electrode substrate has higher rigidity. Thus, the ink jet head having high-precision gaps can be obtained. Also, the silicon oxide film is generally used for interlayer insulating film of semiconductors.
In the above ink jet head, each of the individual electrodes is taken out with a pad on the opposite surface from the liquid chamber substrate in a bonding region between the liquid chamber substrate and the electrode substrate. With this structure, the area for taking out the individual electrodes can be reduced, and the chip size can be reduced accordingly. Thus, more freedom is allowed in packaging, and the packaging procedure can be simplified. Furthermore, the production costs can be reduced.
The above objects of the present invention are also achieved by an ink jet head comprising: a plurality of nozzles through which ink droplets are discharged; a plurality of liquid chambers that respectively communicate with the plurality of nozzles; a diaphragm that is made of a conductive material and forms at least a part of each of the liquid chambers; a first substrate that includes the liquid chambers and the diaphragm; and a second substrate that has electrodes facing the diaphragm. In this ink jet head, the ink droplets are discharged through the nozzles by deforming the diaphragm with electrostatic force generated by a voltage applied between the diaphragm and an electrode facing the diaphragm via a gap; a part of the diaphragm made of a conductive material is electrically separated from each of the nozzles; the electrode facing the diaphragm serves as a common electrode; and the first substrate and the second substrate are bonded to each other on the same single plane, with an insulating layer being interposed therebetween.
With this structure, an ink jet head having high reliability, high bonding strength, and high-precision gaps, can be obtained.
In the above ink jet heat, the second substrate is made of a conductive material, and serves as the common electrode. Because of this, the entire structure can be simplified, and the number of production steps can be reduced. Thus, an ink jet head can be obtained at lower costs.
In the above ink jet head, the first substrate and the second substrate are bonded to each other, with a silicon oxide film being interposed therebetween. At least one of the diaphragm and the common electrode may be made of monocrystal silicon. The first substrate may an SOI (Silicon on Insulator) substrate; at least a part of the diaphragm may be made of monocrystal silicon; the second substrate may be a monocrystal silicon substrate; and the first substrate and the second substrate are bonded to each other, with a silicon oxide film being interposed therebetween.
With the above structure, direction bonding between silicon oxide films or between a silicon oxide film and a silicon material can be performed to bond the diaphragm or the diaphragm material to the individual electrodes or the individual electrode material. Thus, an ink jet head having high-precision gaps can be obtained.
Other objects and further features of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.